Research and development efforts in the photographic materials and paper materials industries often focus on various types of imperfections in a moving coated web. These imperfections may, for example, result from disturbances in the coating process, such as may occur during the sensitization of photographic film. Research and development efforts attempt to isolate, through process modeling, the source of an on-going disturbance-type in a coating process. Coating imperfections of particular interest to the industries are continuous-type imperfections and point-type imperfections. These imperfection types, which can occur in one or more coating levels on a support web, are typically indicative of a disturbance or design related problem in the coating process. An effective on-line imperfection recognition system and method would enable one to discern, characterize and confirm various models of the coating process, thereby determining the disturbance causing such an imperfection. Two significant issues, however, must be addressed by any imperfection recognition system before adequate optical data can be collected from sensitized coatings under examination. First, the system must be able to extract small density changes from the obtainable spatial and temporal noise background. Secondly, the system must provide adequate illumination within the spectral bandwidth of the usable contrast range, while avoiding fogging of any sensitized web.
State-of-the-art efforts to quantize moving web disturbances have most commonly been implemented as laser scanning systems. For example, continuous laser beams are often swept by multifaceted polygon mirror scanners across moving webs of film or paper support, and focused with dedicated optics onto a discrete detector such as a photo-multiplier tube. Various detector configurations enable data acquisition in either a reflective or transmissive mode. Unfortunately, such laser scanner packages can be expensive and typically have limited anomaly detection capabilities.
Specifically, such laser scanning packages are almost universally unable to process data associated with very narrow lines and streaks which may be imbedded in the signal noise background. Also, current laser scan output processing packages, in general, remain less sophisticated than those accompanying state-of-the-art imaging technologies, such as solid state cameras.
More recently, CCD cameras have been proposed for use in scanning webs to detect various types of imperfections. A further CCD camera has been developed with a "time-delay integration" or "time-delay integrating" (TDI) function for a variety of uses as described, for example in U.S. Pat. Nos. 4,922,397 and 5,040,057 as well as in U.S. Pat. Nos. 4,314,275, 4,382,267 and 4,952,809. In these patents, the CCD element charges of each row of CCD photosites or elements of the array of elements is shifted to the next row of elements (while maintaining column alignment) by a line shift clock signal. The line shift clock frequency is synchronized with the incremental movement of a discrete pixel-related image area of the web or object being imaged.
FIG. 1 illustrates the synchronous line shift operation with respect to incremental movement of a web or object through an illuminated imaging region 52 over three successive shift clock time cycles t1, t2, and t3 in respective positions (a), (b) and (c). The M column by N row, CCD element array pattern 50 is fixed in position. Each discrete CCD element of the array pattern 50 images a fixed pixel-related image area of the fixed imaging region 52. For example, a discrete pixel-related image area X in successive, adjacent scan lines 58, 60, 62 of the region 52 is imaged by three CCD elements 64, 66, 68 of Row 1, Row 2 and Row 3 in the same column of the array pattern 50 during each shift clock time cycle. Thus, the scan lines 58, 60, and 62 shift in the direction of arrow 54' with the shift effected by the shift clock in the direction of arrow 54.
Each shift clock time cycle t1, t2, and t3, et seq., includes a charge integration time and a charge shift time. During the charge shift time, the time integrated charges of the CCD column elements in each row are transferred or shifted to the corresponding column CCD elements in the next row in the direction of charge accumulation denoted by arrow 54, so that the charge accumulates as shown in the wave shape of accumulated charge during the illustrated three clock time cycles. When the line shift clock completes tN clock time cycles, the total accumulated N charges in Row N are transferred to a shift register (not shown) and then employed to recreate the image line on a monitor or otherwise processed.
In the prior art example of FIG. 1, it is assumed that the object or a segment of the web 14 is moving incrementally at a predetermined fixed rate in the direction of web movement denoted by arrow 56 within the plane of the imaging region 52. The line shift clock frequency is synchronized to the incremental movement of each pixel-related web stripe area 70 into the adjacent (in the column direction) pixel-related scan line 58, 60 and 62. As a result, the same discrete, pixel-related, web area is successively imaged in the pixel-related image areas of the imaging region 52. The pixel-related area X is shown in the positions at time cycles t1, t2, and t3 reflected onto the same position in web stripe area 70. In this way, charges that are dependent on the light intensity reflected from or transmitted by the same pixel-related web area X' accumulate as it is imaged by a CCD column element in each of the N rows of CCD elements.
Although only a single scan line is highlighted in this example, it will be understood that the shift clock signal is applied to all rows simultaneously and that the accumulated charges over the total N rows is shifted into the shift register from row N during each shift clock time cycle. The result of the synchronization of the line shift clock frequency to the incremental motion is to provide a multiple exposure of the web or object to capture a "stop motion" image and avoid any smearing of the image features. This is referred to short hand as a synchronized TDI CCD camera or TDI camera.
As described in the '267 patent, the synchronized TDI CCD camera (or "TDI imager" as used therein) has application in imaging objects, scenes or moving webs (hereafter web, for convenience) in low light level conditions and, in the context of the field of the present invention, in conditions where the web is moving at such a high rate that not enough photoelectric charge can accumulate in each particular CCD element in any given line or row of CCD elements. As described above, the TDI imager sums the charges accumulated by the M parallel CCD elements in a selected set of N rows of M parallel CCD elements at a TDI row or line shift clock signal frequency synchronized to the web advancement. In other words, as the web advances, a scan line of M pixel-related image areas on the web is successively imaged on the M parallel CCD elements of the N rows, and the accumulated charges for each set of N rows are transferred and summed with new charge each time that the web advances to the next row. As a result, the M.times.N CCD element array may be viewed as a kinear array scanner of M CCD elements imaging a single line of the moving web that is captured N times as the web is advanced.
The TDI function provides an effective gain in sensitivity proportional to the number N of rows of CCD elements contributing to the total summed charges, and random noise attributable to an individual CCD element in any given row is averaged out. In this fashion, enough total charge may be accumulated by the multiple-exposures of the scanned line of the web that a usable contrast image may be created from a display or analysis of the scanned lines of the web. Random noise is reduced approximately in proportion to .check mark.N thereby improving the signal/noise ratio of the accumulated charge signal. A two-dimensional display of sections of webs (i.e., discrete scenes or objects in those contexts) may be created on a suitable display or printed out from a composite number of such line scans forming an image frame.
In order to derive a coherent or focused, two dimensional image display or print from the discrete, accumulated charge line scans, the above referenced patents all emphasize the need to synchronize (or derive) the CCD row shift clock signal for transferring the charges of each CCD element in each row of M CCD elements to corresponding CCD element in the next row to the incremental movement of the scanned line of the web. In the '057 patent, the TDI imager disclosed therein may also be selectively switched from this "synchronized mode" to operate in a conventional video camera raster scan mode, or the shift clock signal may be switched to an internal clock, in order to operate the TDI imager in a stationary scan mode or in a mode where an image is "grabbed". This process is described in regard to use of a non-TDI CCD camera and strobe lamp or web position sensor in an earlier U.S. Pat. No. 4,896,211. In either the internal clock or raster scan mode, the TDI imager may be used to derive images of stationary objects, scenes or documents or to initialize or align the TDI imager with respect to a stationary web.
The operation of the synchronous TDI imager provides clear frame images of moving webs, particularly to image specks or holes or other discrete, non-continuous blemishes or imperfections at low light levels which would be insufficient for a non-TDI CCD camera. This capability eliminates the need for strobe lighting and provides full frame images without substantial web image overlap from frame to frame. These characteristics were viewed as desirable in the above-listed patent descriptions. In fact, it is represented that the TDI CCD camera is not optimally usable absent this capability.
A need continues to exist today in the photographic and paper materials industry for a more effective and less expensive technique to extract and characterize imperfections from background data including inherent noise variations, and particularly low-level, narrow continuous-type imperfections in a moving coated web.